Nitrogen-bearing ferrochromium



United States NITROGEN-BEARING FERROCHROMIUM Earle R. Saunders, Grand Island, and William L. Harbrecht, Kenmore, N. Y., assignors to Union Carbide and Carbon Corporation, a corporation of New York No Drawing. Application October 1, 1953, Serial No. 383,708

4 Claims. (Cl. 75--28) This invention relates to nitrogen-bearing ferrochromium and has for its principal object the provision of a ferrochromium pellet of low carbon content and high nitrogen-to-carbon ratio.

The value of nitrogen as an addition agent in the manufacture of chromium steels has been recognized in the steel industry for many years. The principal effect of nitrogen as an alloying addition is to impart a marked refinement of the grain structure of the high-chromium steels. This grain refinement is reflected in an improvement in the physical properties of strength and toughness without significantly increasing the hardness which is associated with brittleness. Nitrogen has also been shown to be advantageous in promoting the formation of the austenitic phase, and therefore, can be used as a partial replacement for the austenitizing elements in critical supply. Nitrogen is analogous to carbon with respect to grain refinement benefits, but has the important advantage of not producing the hardening characteristics which are always associated with the introduction of carbon. Furthermore, nitrogen in high-chromium steels has little effect upon the corrosion resistance and the resistance to oxidation at high temperatures in contrast to the recognized detrimental effect of carbon on these important properties.

To attain these benefits of nitrogen, it has been the practice to introduce nitrogen into chromium steels during the finishing period of a heat by adding a nitrogenbearing ferrochromiurn alloy. The alloy most often used contains about 65% to 70% chromium; less than 1% silicon; 0.75 to 2% nitrogen; and 0.1 maximum carbon. The nitrogen-to-carbon ratio of the alloy is generally not more than 9 to 1.

The alloy just described cannot be produced directly on a commercial scale but is manufactured by adding a nitrided ferrochromium to a molten conventional low carbon ferrochromium generally at the end of a heat. The nitrided ferrochromium used must itself be prepared from comminuted low carbon ferrochromium of less than about A inch particle size. The comminuted material is heated in trays in a mufile furnace in the presence of air, a six hour heating at 1100 C. being required. This treatment results in a sintered product containing about 4.5% to 6% nitrogen. The sinter is broken up into suitable size and is then added to a molten, low carbon ferrochromium as above indicated. The molten alloy is then cast into chills in conventional manner and broken up into convenient sizes. It is a hard, dense product.

In recent years, the general trend in the steel industry engaged in the production of high-chromium steels has been in the direction of progressively decreasing the carbon content in order to obtain improved resistance to corrosion. In attaining this goal of low-carbon contents on a commercial basis, new techniques for furnacing have been developed and new types of ferro-alloys have been required and produced. In order to capitalize on the low-carbon contents achieved during decarburization and 2,797,156 Patented June 25, 1957 thereby produce steel with a minimum carbon content, it is necessary to restrict the amount of carbon introduced during finishing in the form of the ferro-alloys, principally, of course, in the ferrochromium alloy. To meet this need, ferrochromium with a very low carbon content (0.025% maximum carbon) is now being produced commercially in pellet form by solid-phase, vacuum-decarburization of high-carbon ferrochromium (United States Patents Nos. 2,473,019, 2,473,020 and 2,473,021).

For the manufacture of chromium steels of very low carbon content, the conventional nitrogen-bearing ferrochromium alloys are not entirely satisfactory. To begin with their nitrogen-to-carbon ratio is low, so that the addition of a desired quantity of nitrogen by use of the alloys is accompanied by the addition of an undesirably large quantity of carbon. Furthermore, because of the low silicon content of the alloys their melting point is rather high and this makes for longer solution time, which in turn gives greater opportunity for carbon pick-up by the steel from the electrode of the furnace.

The invention comprises a nitrogen-bearing ferrochromium alloy of very low carbon content and a high nitrogen-to-carbon ratio combined with a relatively high sili- .50% to 90% chromium; not more than 0.25% carbon;

1% to 6% silicon; 0.5 to 7% nitrogen; the remainder iron, the nitrogen-to-carbon ratio being at least 20 to l. A more restricted range of composition of the ferrochromium is: 60% to chromium; not more than 0.1% carbon; 4% to 6% silicon; 0.75% to 7% nitrogen; remainder iron, the nitrogen-to-carbon ratio being at least 30 to l.

The nitrogen-bearing pellets of ferrochromium of the invention are prepared by exposing compacted pellets of low carbon ferrochromium to a nitrogenous atmosphere at a temperature between 900 C. and 1400 C. for a period of time sufficient to permit the desired degree of nitriding. Although it is entirely feasible to nitride pellets produced by the methods of the patents referred to above after they have been removed from the vacuum furnace simply by reheating the pellets to the proper temperature range in a suitable furnace in an air or a nitrogen-enriched atmosphere, it is preferred to accomplish nitriding without prior removal of the pellets from the vacuum furnace.

An advantage of the latter procedure is that the amount of nitrogen required to give the desired nitrogen content in the pelleted ferrochromium can be admitted into the furnace at the end of the decarburization period and a much closer control of the operation and the product may be attained. Thus, the furnace is charged with pellets composed of comminuted high carbon ferrochromium, oxidant material and binder, the charge is decarburized by heating at a temperature below about 1400 C. and subatmospheric pressure, and after decarburization has progressed to the desired degree, nitrogen is admitted. When the nitrogen is absorbed, as indicated by the drop in pressure in the furnace, the charge is cooled in the normal manner and the product is ready for use.

The following examples illustrate the preferred method and show the precise control of composition which is obtained.

Three heats, A. B, and C were made, aiming at 0.025% carbon maximum and 0.75 1.50% and maximum percent nitrogen, respectively. A mix of the following ratio was prepared, pelleted, dried and decarburized in a vacuum furnace:

Pounds Comminuted high-carbon ferrochromium 400 Silica sand 59 Glucose 9 /2 Water 27 In heats A and B, after the decarburization was complete, nitrogen to give the desired content was introduced into the furnace at 1350 C. By the time the furnace had cooled to 1050 C., nitrogen absorption was almost complete, the pressure having fallen to 0.3 inch mercury absolute. The charge was then cooled in the usual way. In heat C, nitrogen was admitted to the furnace at 1150 C., until the pressure was 2%. inches mercury above atmospheric. For the first three hours the heat of nitriding resulted in a temperature rise reaching 1220 C. After six hours, the rate of nitrogen absorption had fallen to 20 cu. ft. per hour, and the furnace was cooled in nitrogen. The results of these heats follow:

Percent Carbon Percent Nitrogen Obtained Aimed For Obtained 0. 75 0. 74 1. 50 Maximum 5. 95

A typical composition of the nitrided ferrochromium pellet of the invention is: 64% chromium; 5% silicon; 0.75% to 6% nitrogen; 0.02% carbon, the nitrogen-tocarbon ratio being at least 100 to 1.

Many tests of the nitrogen-bearing ferrochromium pellets of the invention have shown them to be a superior addition for the manufacture of nitrogen-bearing low carbon chromium steels. In addition to the fact that much less carbon is introduced per unit of nitrogen added, the nitrided pellets possess an advantagewith respect to rate of solution in a steel bath. The unique physical form of the pellets, relatively porous in nature, combined with the lower melting point due to the presence of high silicon in the alloy results in more rapid solution in steel than with conventional, dense ferrochromium. Since changes in bath chemistry occur due to transfer of oxidizable elements from metal to slag during the finishing of a steel heat and a steady increase in carbon content results from pickup from the electrodes, a saving in time is very important from the standpoint of control of composition. Obviously, a saving in time is highly desirable since it represents a reduction in the cost of steel making. In addition to decreasing the melting point and thereby promoting rapid solution in the steel bath, the silicon in the nitrided pellets is advantageous in contributing to the maintenance of the reducing conditions necessary during the finishing period of a high-chromium steel heat. Due to the protective effect of the more readily oxidizable silicon in the alloy, improved recovery of chromium is realized from the pellets. Part of the contained silicon is recovered in the steel bath as an alloying element, and thereby is of value in meeting the specified silicon concentration for high-chromium steels. The higher nitrogen content possible in these pellets is a distinct advantage for the manufacture of certain grades of chromium and chromium-manganese steels where the addition of new chromium is limited due to high scrap charges.

Illustrative of the results obtained by the use of the pellets of the invention in a one-ton arc furnace heat, nitrided ferrochromium pellets were used to introduce nitrogen into a 16% chromium, 16% manganese, 1% nickel stainless steel containing 0.12 to 0.16% nitrogen. The pellets were of the type obtained in Run B above, containing about 1.5% nitrogen. They were added in the furnace through the finishing slag shortly before tapping the heat. The recovery of nitrogen from the pellets was calculated to be 87%, which is considered normal performance as compared to the conventional nitrogenbearing ferrochromium. Because of the receptivity of high-manganese steel for nitrogen, it is believed that nitrided pellets saturated with nitrogen (about 7%) could be used efficiently for these steels. Since economic melting practice for these steels dictates high-chromium scrap charges, the addition of new chromium is limited, which makes the high-nitrogen content of the pellets very attractive for this type application.

Illustrative of the results of the use of the pellets of the invention in making chromium steels of minimum carbon content, two one-ton arc melted heats were made with a 25% scrap charge to produce a 21.5% chromium steel with an 0.11% nitrogen and a 0.03% maximum carbon specification. The nitrogen was introduced as 2% nitrogen grade nitrided ferrochromium pellets shortly prior to tapping. The indicated recovery of nitrogen was about 90% in each heat, and the carbon contents of the tapped metal were 0.023% and 0.026%.

What is claimed is:

1. A compacted, uniformly porous, cohesive aggregate of ferrochromium containing 50% to 90% chromium; not more than 0.75% carbon; 1% to 6% silicon; 0.5% to 7% nitrogen; the remainder iron, the nitrogen-towarbon being at least 20 to 1.

2. A compacted, uniformly porous, cohesive aggregate of ferrochromium containing 60% to 75% chromium; not more than 0.1% carbon; 4% to 6% silicon; 0.75% to 7% nitrogen; the remainder iron, the nitrogen to carbon ratio being at least 30 to 1.

3. A pellet composed of a compacted, uniformly porous, cohesive aggregate of ferrochromium containing 60% to 75% chromium; less than 0.1% carbon; 1% to 6% silicon; 0.75% to 6% nitrogen; the remainder iron, the nitrogen-to-carbon ratio in said ferrochromium being at least to l.

4. Method of producing nitrogen-bearing ferrochromium of a nitrogen-to-carbon ratio of at least 20 to 1 which comprises compacting a finely divided high carbon ferrochromium with an oxidant and a binder, heating said aggregate in a furnace at subatmospheric pressure to an elevated temperature below the fusion point of any part of said aggregate whereby to decarburize the same and then admitting nitrogen in an amount determined by the degree of nitriding desired into the furnace, and holding said aggregate at a temperature of at least 900 C., but not over 1400 C. until 0.5% to 7% of said nitrogen has been taken up by said aggregate. 

1.A COMPACTED, UNIFORMLY POROUS,COHESIVE AGGREGATE OF FERROCHROMIUN CONTAINING 50% TO 90% CHROMIUM; NOT MORE THAN 0975% CARBON; 1% TO 6% SILICON; 0.5% TO 7% NITROGEN; THE REMAINDER IRON, THE NITROGEN-TO CARBON BEING AT LEAST 20 TO
 1. 